Lecture 8 February 7 2003 R.Jones Chapter 10
DNA and the Genetic Code
1.Hershey and Chase and bacteriophage. See module 10.1 of text.
2. DNA consists of the sugar deoxyribose, a phosphate group and the four nitrogen-containing bases, adenine (A), guanine (G), thymine (T) and cytosine (C). A and G are purines and T and C are pyrimidines.
3. Chargaff found that in any given cell A = T and G = C. A& G and T & C are never equal. These Chargaff Rules were exploited by Watson and Crick in the construction of their model of DNA.
4. Watson and Crick proposed that the DNA molecule was a double helix of antiparallel DNA strands held together by hydrogen bonding betwee A and T and G and C.
5. The DNA strand replicated by copying two new strands on the original parent strands by specific base pairing A =T and G = C. The process of strand duplication is referred to as semi-conservative replication. Semi-conservative because after replication, two new daughter strands consist of half of the old strand plus a new copy formed as a result of specific base pairing. The enzyme DNA polymerase catalyzes the process.
6. The Watson and Crick model accounted for the faithful replication of genetic information because new DNA strands are identical to the parent strand. Editing of the DNA strands ensures that no errors are made. Any errors, e.g., insertion of the wrong base are corrected. If correction not made a mutation is caused.
7. Jacob and Monod, two French researchers discovered messenger RNA (mRNA) and the Central Dogma of Molecular Biology was born. Dogma states that DNA gives mRNA, called transcription and mRNA gives Proteins by translation.
8.mRNA similar to DNA except: single stranded molecule formed from DNA by enzyme RNA polymerase. Contains ribose not deoxyribose, uracil (U) instead of thymine (T) (so A base pairs with U when RNA is synthesized).
9.mRNA carried the genetic code into the cytoplasm. Discovered that the genetic code is written in a language that is composed of triplets of bases. The four bases (AUGC) can give a maximum of 64 combination of triplets (see Figure 13.7 of your textbook).
10.When mRNA is translated into protien two other types of RNA are involved, ribosomal RNA (rRNA found in the ribosome) and transfer RNA (tRNA).
11. Whereas the mRNA carries information about the order of amino acids in proteins in the form of the triplet code, tRNA is specific for each amino acid and recognizes the triplet code with a three base anticodon. Each tRNA has an amino acid attachment site as well as an anticodon. The triplet code of the mRNA and the anticodon of the tRNA base pair thus ensuring that the correct amino acid is inserted into the groing protein chain.
12. Translation occurs in the cytoplasm. tRNAs bring amino acids to the correct codon in the mRNA and the ribosome causes a chemical bond to form between each adjacent amino acid, causing the protein chain to elongate (see Figure 13.14).